It is known in the art that a silver diffusion transfer process may provide a positive silver transfer image by development of the latent image provided by a photosensitive silver halide emulsion by exposure and, substantially contemporaneous with development, a soluble silver complex is obtained by reaction of a silver halide solvent with unexposed and undeveloped silver halide in the emulsion. The resultant soluble silver complex is at least in part transported in the direction of a suitable print-receiving element and the silver of the complex is there precipitated on silver-precipitating nuclei to provide the requisite positive silver image formation.
As set forth in U.S. Pat. No. 2,698,236 the array of silver atoms precipitated in the image-receiving element is influenced by the size of the silver-precipitating nuclei. If the silver precipitating nuclei are relatively small, the silver deposited thereon will be of a corresponding size and, therefore, generally red in color which is undesirable. Clusters or galaxies of silver-precipitating nuclei possessing a diameter directly proportional to the mass of image silver to be precipitated therein are disposed in the image-receiving layer to cause silver to precipitate in association with silver-precipitating nuclei clusters with a required density and of a size directly related to the physical parameters of the clusters, thus providing the desired black silver image.
Nobel metal silver-precipitating nuclei are known to the art. U.S. Pat. Application Ser. No. 649,201 filed Jan. 14, 1976 discloses a method of forming such nuclei by the reduction of a noble metal salt or complex. U.S. Pat. No. 3,647,440 also discloses noble metal silver-precipitating nuclei obtained by reducing a metal salt in the presence of the colloid with a reducing agent having a standard potential more negative than -0.30.
The present invention provides a novel method for forming the above-mentioned clusters or galaxies of noble metal silver-precipitating nuclei.
Copending application Ser. No. 649,201 also discloses the dialyzing of the colloid formed by the reduction of the noble metal salt before coating. The aforementioned application also discloses the precipitation of the noble metal by adjusting the pH of the colloid to an alkaline level whereby the nuclei can be separated, washed, redisposed in a polymer and coated.